Interesting project from Randizzle88 from Remington 870 Forum:
Well hey guys I found out for the 870 express 6+1 tactical shotgun, the Remington 4+1 barrels will not work. For some of you guys looking to fit a vented barrel on a 870 express 6+1. You will find nobody makes one where the mounting point is on the 6+1 870s. So here is what I did to make it work. I bought a 28 inch vent rib barrel 4+1 and made a spacer tube to fit a 6+1. I used a chop saw to cut multipurpose 6061 aluminum tube I got from the hardware store 1-1/4″ OD. Make sure it cuts a perfect 90 and slow cuts. Then I hand sanded the inside of the aluminum pipe to fit the magazine tube perfect.